This invention relates to a railway car brake control system and particularly to such a system that blends the electric brake and the fluid brake, such as an air brake, to make up for a deficiency in the power of the electric brake with the power of the fluid brake when the electric brake power is not enough for the brake power command signal.
When the brake control changes from electric brake system to the fluid brake system, the total brake power (the sum of the electric brake power and the fluid brake power) usually becomes lower, due to the hysteresis effect that the electric-fluid pressure converting valve has and the response delay of the fluid pressure. This invention is concerned with preventing this lowering of the brake power.
Following is an explanation of known brake control systems using FIG. 1 to FIG. 5. In FIG. 1, 1a is the brake power setter, such as an electrical digital to analog transducer.
The brake power setter 1a converts the three-bit digital signal, which is generated by the brake valve BV (not shown) via train line wires SB1, SB2, SB3 into an analog signal, depending on the brake power command signal, E. The electromagnetic valves MV1, MV2 and MV3 are supplied with a pressurized fluid which is adjusted according to the car weight, and which comes from the weight system 2b that is connected to the pressurized fluid source. MV1, MV2 and MV3 supplies and releases the suitable pressurized fluid, which is determined by the car weight, to the diaphragm chambers C1, C2 and C3 of the operational relay valve 4, after being magnetically operated in accordance with the previously-described three-bit digital signal.
The electric brake system EB functions according to the brake power command signal, which comes from the brake power setter 1a, to provide electric brake power to the car. The first detector 11 detects actual electric brake power that is effective from the motor current etc., and outputs it as the electric brake feedback signal Ei. The amplifier circuit 13 inputs the electric signal Ei, which comes from the 1st detector 11, and outputs Eo, here Eo=K (Ei+A), to convert it to a control level. Here, K is the amplification ratio and A is the set value. In the electropneumatic converter valve EP, the solenoid SOL produces electromagnetic force according to output signal Eo of amplifier circuit 13. Under the influence of this force, the valve rod 51 opens the supply valve 52. Supply pressure is thus connected to the valve outlet via chamber 53. This fluid pressure in the output chamber 53 is effective to force the balance piston 54 in an upward direction opposing the force exerted by solenoid SOL. When the solenoid force and the fluid pressure force become balanced, two-way valve rod 51 engages valve 52, which is in turn engaged with the supply valve seat, so that output chamber 53 is kept at that fluid pressure.
Since the output chamber 53 is connected to the diaphragm plate chamber C4 of the operational relay valve 4, the electropneumatic converter valve EP supplies or releases pressurized fluid to the diaphragm plate chamber C4, depending on the signal Eo, which is the output of amplifier circuit 13. The pressurized fluid which is supplied to the diaphragm plate chambers C1, C2, C3, through the operational relay valve 4, works to raise the valve rod 41 using the diaphragm plate pistons S1, S2, S3.
But, the pressurized fluid, which is supplied to the diaphram plate chamber C4, acts in the opposite direction. In other words, the operational relay valve 4 subtracts the output signal of amplifier circuit 13, which is the effective electric brake power signal derived from the brake power command signal; hence, it can be called an "operator".
In the case where the electric brake power is insufficient to satisfy the brake power command signal effective at the operational relay valve 4, the valve rod 41 opens the supply valve 42. Therefore, the fluid pressure of the output chamber rises and this pressure pushes down the balance piston S5. When both the up and down forces of the balance piston are equalized, the inlet-outlet valve rod 41 engages the supply valve 42, which in turn is engaged with its seat by a light spring, so that the output chamber 43 is kept at that fluid pressure. The output chamber 43 of the operational relay valve 4 is connected to the brake cylinder BC, which is part of the fluid brake system, and the brake cylinder is supplied and drained of pressurized fluid under control of the operational relay valve 4. In other words, in the above-described brake system, the deficiency in the electric brake power of the brake power command signal is supplemented with the fluid brake power of the brake cylinder BC. By the way, as shown in FIG. 2, the electropneumatic pressure converter valve EP does not function immediately as signal Eo increases from 0, the delay being due to the frictional resistance inside the valve. This delay is defined by the value W (FIG. 2).
So, in amplifier circuit 13, the bias signal A, which is equivalent to the point W, and which is a set value, is input together with the output signal Ei of the first detector 11. Amplifier output Eo is found by the equation Eo=K (Ei+A), as shown in FIG. 4. The amplifier circuit shown in FIG. 3, includes an input resistance R1 and a feedback resistance R2, the ratio (R2/R1) being the amplification ratio K.
Also, the electropneumatic pressure converter valve EP, as shown in FIG. 2, has hysteresis, in which the output pressure rises from point (W) to point (X) with an increase in its input, and falls from point (Y) to point (Z), via point (Y), with a decrease in the input; wherein the output pressure, when the input is decreasing, is higher than the output pressure when the input is increasing. Also, the change rate of the output pressure of the electropneumatic converter valve EP, i.e., the fluid brake power supplemental rate generally is delayed from the change rate of the electric brake power, i.e., the change rate of the output signal Eo of amplifier circuit 13. Therefore, in the above-described known brake control system, the sum of the electric brake power and the fluid brake power, i.e., the total brake power, becomes less during and after the transition from the electric brake to the fluid brake. This problem will be explained using FIG. 5. In FIG. 5a, the electric brake system EB is effectively working in accordance with the brake power command signal E. In such a state, when the car speed begins to decrease, the effectiveness of the electric brake power begins to decrease. This electric brake power is detected by the first detector 11, and the output signal Ei of the first detector 11 decreases until it becomes 0, as shown in FIG. 5b. However, the output signal Eo of amplifier circuit 13 does not decrease to 0 because of bias signal A, as shown in FIG. 5c. Even after the electric brake power totally loses its effect, this (K.A) is still input into the electropneumatic converter valve EP. Because of its hysteresis, the electropneumatic converter valve does not respond immediately to the decrease of the signal Eo. As shown in FIG. 5d, after a small delay, the output fluid pressure is reduced; and even after the electric brake completely loses its effect, there is still a residual pressure because of the bias signal A. In other words, while the electric brake power is decreasing, in the middle of the process, the output fluid pressure of the electropneumatic converter valve EP is higher than the fluid pressure corresponding to the actual electric brake power. And also, even if the electric brake power becomes 0, the electropneumatic converter valve EP still continues to output. This output fluid pressure of the converter valve EP will be subtracted from the fluid pressure, which is equivalent to the brake power command signal E at the operational relay valve 4. Therefore, the fluid brake power delays to begin to rise and therefor only reaches a lower level, as shown in FIG. 5e. Therefore, as shown in FIG. 5f, because of the reduced supplemental fluid brake power, the total brake power becomes lower when the electric brake is supplemented by the fluid brake, and as a result, the riders feel unpleasantness; and even after the braking power changes totally to the fluid brake, it still does not satisfy the total brake power, and the total stopping distance becomes longer. To solve this problem, especially the deficiency of the brake power, not during the transition from the electric to fluid brake, but after the transition, another known brake control system has been disclosed which is shown in Japanese Patent No. 56-148105. This second known brake control system, as shown in FIG. 6, was changed from the FIG. 1 style system by adding the second detector 12, which detects and outputs the total loss of the effect of the electric brake power, and the output Eo of amplifier circuit 13 is forced to 0 according to the output signal of the second detector 12. Also, the construction of amplifier circuit 13 is changed, as shown in FIG. 7. In other words, the second detector 12 gives an output only when the electric brake power is 0; and according to the output signal of this second detector, a normally open contact S is closed and the feedback resistance R2 is short-circuited. Hence, the output Eo of the amplifier 13 becomes 0. In other situations, the second detector is not outputting, so that the contact S is in the open position. Hence, output Eo of amplifier circuit 13 follows the equation Eo=K (Ei+A). Therefore, as shown in FIG. 8a, when the electric brake is working effectively for the brake power command signal E, if the output signal Ei of the first detector 11 becomes lower (as shown in FIG. 8b), because of a decrease in the electric brake effectiveness as the car speed decreases, the output signal Eo of amplifier circuit 13 concurrently falls as in FIG. 8c. When Ei becomes 0, Eo also becomes 0, and the output fluid pressure of the electric-fluid pressure converting valve EP begins to fall with a delay, and suddenly it drops to 0 when Eo becomes 0. Therefore, although the output of the operational relay valve 4, i.e., the supplemental brake power, begins to rise with a delay, as in FIG. 8e, it rises more at the end to fill up the ordinary deficiency of FIG. 1. Therefore, as shown in FIG. 8f, the total brake power, after the transition to the fluid brake, does not have any deficiency. However, the total brake power during the transition from the electric brake to the fluid brake does not show any improvement for the deficiency of the supplemental fluid brake power; and, just like the known system of FIG. 1, the total brake power becomes lower and the riders feel unpleasantness and the stopping distance becomes longer.